The invention generally relates to control systems and methods for optical amplifiers. More specifically, the invention relates to systems and methods that control the gain provided to a signal travelling in a fiber plant.
Many types of optical amplifiers exist in the optical telecommunications field. A dominant player is the EDFA (erbium doped fiber amplifier). As is well known, an EDFA utilizes a doped section of fiber that is pumped by one or more optical pumps. Because the properties of the doped fiber section are well known and because the amplification occurs in the doped fiber section, the gain properties of a given EDFA are easily controlled to produce a desired gain.
Another type of optical amplifier is the distributed amplifier. Examples of distributed amplifiers include Raman and Brillouin amplifiers. As is also well known, distributed amplifiers perform distributed amplification over a relatively long segment of fiber. For example, a typical Raman amplifier may distribute amplification over 20km of fiber.
The optical fiber used by such distributed amplifiers to amplify the signal is typically the existing optical fiber to which the optical pumps are connected. Distributed amplifiers depend upon the existing fiber plant as the amplification medium in which the amplification occurs. In other words, the amplification medium is not part of the amplifier itself, i.e. not engineered by the amplifier manufacturer.
Existing fiber plants vary widely in their respective properties. Specifically, the gain properties are not constant from one fiber plant to the next fiber plant. The amount of gain produced by a distributed amplifier will change because the amount of gain of a distributed amplifier varies as a function of the fiber plant gain properties.
More specifically, the distributed gain properties such as the Raman gain properties of the fiber plant are, among others, dependent on the fiber type (NDSF, NZDSF such as Coming LS, LEAF, Lucent Truewave(copyright) Classic, RS, Plus, DSF etc.), the loss of the fiber (dependent on aging, fiber type), discrete losses such as splices, connectors, patch panels, etc. These parameters may vary considerably from fiber plant to fiber plant.
Thus, a need exists for detecting these parameters so that a distributed amplifier such as a Raman amplifier and thus the optical transmission system will work at the desired gain level.
In general, the invention includes systems and methods for determining the distributed gain properties of a fiber plant and controlling an optical pump according to the determined gain properties. By applying the inventive techniques, the gain of a distributed amplifier system using a particular fiber plant can be precisely controlled to produce a desired gain value.
The inventive apparatus includes a detector, a controller and a memory device. The detector measures signal levels at various pump power levels. The controller calculates gain for various pump power levels to construct a gain profile or transfer function which may be, for example, gain as a function of pump power. The controller may store the gain profile in the form of a look-up table in the memory device.
By accessing the look-up table stored in memory device or by utilizing a calculated transfer function, the controller can determine the output power corresponding to a desired amount of gain. The controller can then control the optical pump to provide this output power and thereby produce the desired amount of gain for the signal.
The invention may be applied to single wavelength or multi-wavelength systems. In a multi-wavelength system such as a wavelength division multiplexed system, the invention may also include a filter that is used to filter out wavelengths outside the operating range. The filter may be used in conjunction with gain calculation that sums the gain for all of the channels or wavelengths of the system. Other types of gain calculations may also be used by the invention to determine an appropriate gain transfer function for the fiber plant.
In particular, the inventive methods may include providing distributed amplification having a desired gain for a signal travelling in a fiber plant, including increasing an output power of an optical pump operatively connected to the fiber plant and operable at a wavelength selected to provide distributed gain to the signal travelling in the fiber plant; measuring a signal level of the signal travelling in the fiber plant at an increased output power of the optical pump increased by the increasing step; calculating a function relating the output power of the optical pump and the signal gain of the signal travelling in the fiber plant; storing the function calculated by the calculating step relating the output power of the optical pump and the signal gain of the signal travelling in the fiber plant; and controlling the output power of the optical pump according to the stored function to provide the desired gain.
Furthermore, the inventive apparatus may control an optical pump to provide a desired gain for a signal travelling in a fiber plant, and including a detector operatively connected to the fiber plant, a controller operatively connected to the detector and to the optical pump; a memory device operatively connected to the controller; the controller controlling the optical pump to increase output power of pumping light injected into the fiber plant; the detector detecting a signal level of the signal travelling in the fiber plant at an increased output power of the optical pump increased by the controller; the controller receiving the signal level from the detector; the controller constructing a transfer function relating the output power of the optical pump and a signal gain of the signal travelling in the fiber plant; and the controller controlling the output power of the optical pump according to the transfer function to provide the desired gain.
The inventive techniques may be applied whenever a distributed amplifier, such as a Raman amplifier, is installed. Alternatively, a calibration switch or other command signal input can be utilized to trigger the inventive methodologies.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.